Have they inspected the exhaust manifold for any evidence of a plastic bag possibly getting trapped on it?

Never heard of an SPI..... ever.

There are plenty of shops that can do MUCH, MUCH better than a "BMW factory fresh" paint job....thats a laugh. BMW paint quality sucks. Indeed, the hardest part is reproducing the degree of orange peel so that an experienced eye cannot tell which panel is in fact better than factory.

No need to have it repaired "at BMW". You can have it repaired wherever you want. They are financially liable for your loss, just as if they had driven into your car- you are not required to allow them to fix it.